Silicone release coatings or compositions are well known and the subject of many publications and patents. They are useful in many applications where one requires a relatively non-adherent surface. In such applications release compositions are coated onto a substrate, and are caused to be cured. A particularly useful application is the coating of paper, film, and other materials that are used among other applications by providing non-stick surfaces for pressure sensitive adhesive labels, decorative laminates, and transfer tapes.
Silicone release compositions have been used in large volumes for many years. Silicone polymers and copolymers have been used extensively in release compositions because they are inherently low in surface energy. The silicone polymers and copolymers (sometimes referred to as polyorganosiloxanes) used in the prior art for making release compositions can be radiation cured or thermally cured. In the beginning the silicone release composition of choice was the thermal solvent diluted condensation cured release coating system. Silicone technology has progressed from thermal condensation curable solvent-based release coating systems to thermal addition curable solvent-based release coating systems and then to thermal addition curable solventless release coating systems and radiation cured release coatings. The advantages of using solventless, radiation curable, or thermally curable release compositions are well known to the person skilled in the art of release coatings.
Thermal addition curable solventless compositions have become the dominant release coating compositions for delivering pressure sensitive adhesive products to the market place. These markets include pressure sensitive label, graphic arts, tapes, prepreg, and medical, to name a few. Accordingly, to be effective, a thermal addition curable solventless silicone release system must be versatile and meet minimum converter demands.
It is desirable that release coated papers and films have a release force which is low enough to enable the release backing sheet to be easily removed from the coated substrate, but not so low that the release backing sheet will become prematurely separated from the coated surface by forces normally encountered in handling and processing. “Release force” is defined as the amount of force required to peel or separate the release coated substrate from the adhesive or facing.
Conventional thermal addition curable solventless silicone release coatings have a high coefficient of friction compared to condensation curable solvent-based silicone release coatings. The higher coefficient of friction imparts a problematic characteristic referred to as “grab” wherein the coated substrate tends to stick or grab. Due to the differences in the both physical and functional chemistry between condensation curable solvent-based systems and thermal addition curable solventless systems, the thermal addition curable solventless system produces a tacky grabby feel as opposed to the waxy slippery feel of the condensation curable solvent system. It has been the desire of the Pressure Sensitive Adhesive (PSA) industry to have a thermal addition curable solventless system that gives the waxy slippery feel of a condensation curable solvent-based system.
In order to overcome the “grab” problem, associated with thermal addition curable solventless compositions, current technology uses very high molecular weight nonreactive polymers to reduce the coefficient of friction or grab. These high molecular weight polymers tend to bloom or migrate to the surface to impart additional slip however they also tend to cause converting problems.
During manufacture and storage of the coated sheets prior to use, the free nonreactive silicone oil that has bloomed to the surface is inevitably released from the coated sheet onto the surface of another coated or uncoated sheet leaving discrete patches of free nonreactive silicone oil on the sheet surface. When, for example, the coated sheet is stored in a front surface to back surface contact manner, as in a typical roll, some of the free nonreactive silicone oil (typically high molecular weight silicones) on the front silicone coated surface of the sheet will be transferred to the back (typically uncoated) surface of the roll.
In practice, this free nonreactive silicone oil contamination can be a drawback. For example, when the coated sheet traverses over idler rolls during subsequent processing steps (e.g., during label application), the idler rolls can develop a buildup. This can result in uneven tension on the sheet web or loss of alignment. Related problems arise when printing on the resulting release coated sheet or when labels are applied to the sheet. The ability of printing inks and solvents to adhere is impaired by the presence of free nonreactive silicone oil on the surface to be printed. Moreover, loss of alignment due to the sheet's slip characteristics can lead to a high reject rate during printing, particularly where multiple printing passes are used and in cases where proper alignment is critical for formation of an integrated image.
By having a thermal addition curable solventless system that produces a coefficient of friction similar to a condensation curable solvent-based system, a more complete cure off the coater will be achieved, thereby providing a more stable release coating when compared to thermal solvent condensation cured release coating systems.
Accordingly, there exists a need in the art to provide a thermal addition curable solventless silicone release composition with a lower coefficient of friction and lower slip. A need also exists to provide a thermal addition curable solventless silicone release compositions with similar slip and feel properties to a condensation curable solvent-based silicone release composition.